introduce SVE_ATTR macro

- Add a preprocessor guard around SVE-specific function attributes
- Encapsulate the +sve attribute behind OMPI_MCA_OP_SVE_EXTRA_FLAGS, ensuring
  that only builds which detected and enabled compiler SVE support will compile with
  SVE-targeted code paths.
- Simplifies later code by using SVE_ATTR in function declarations instead of
  repeating the attribute clause.

Signed-off-by: Marco Vogel <marco.vogel@fernuni-hagen.de>

Author: vogma

ompi/mca/op/aarch64/op_aarch64.h

@@ -24,6 +24,12 @@
 
 BEGIN_C_DECLS
 
+#if defined(OMPI_MCA_OP_SVE_EXTRA_FLAGS)
+#define SVE_ATTR __attribute__ ((__target__ ("+sve")))
+#else
+#define SVE_ATTR
+#endif
+
 /**
  * Derive a struct from the base op component struct, allowing us to
  * cache some component-specific information on our well-known